Apparatus for time-delay measurement



Jan. 15, 1957 L. FRANK APPARATUS FOR TIME-DELAY MEASUREMENT Filed Dec.7, 1949 3 Sheets-Sheet l Jan. 15, 1957 Filed Dec. 7, 1949 Z vous R. L.FRANK l APPARATUS FOR TIME-DELAY MEASUREMENT 3 Sheets-Sheet 2 ATTORNEYJan. 15, 1957 R. L.. FRANK A 2,778,011

APPARATUSFOR TIME-DELAY MEASUREMENT Filed Dec. '1, 1949 s sheets-sheet szug. 6i.

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L r L f Ji 98 9e @2f/S l ca/Nc/oE/vcf FM TER' l cvecw/T l 1 l INVENTR.FasL-RTL FRA/VK ATTORNEY United States Robert L. Frank, New York, N.Y., assignor to Sperry Rand Corporation, a corporation of DelawareApplication December 7, 1949, Serial No. 131,677

Claims. (Cl. 343-103) This invention relates to automatic synchronizercircuits and particularly to completely automatic apparatus forindicating the time-delay between a pair of recurrent pulses.

Automatic systems for indicating the time-delay between a pair ofrecurrent pulses are known in the prior art. Generally, such systems areactuated by the amplitude of the control pulses and hence are adverselyalected by random noise signals and by changes in the amplitude or waveform of the control pulses so that the synchronized pulses do not alwaysoccur in precisely the same time relation with reference to therespective control pulses; e. g., in Loran systems pulses of carefullycontrolled shape are transmitted in predetermined time relation from twolocations and receiving equipment on a mobile craft is employed toreceive and detect the pulses and to provide a measure of the time-delaybetween the pulses received from the two locations, from which ahyperbolic line of position of the craft is ascertained. The accuracywith which the lineof position is determined depends, of course, uponthe accuracy of the timedelay measurement. This, in turn, has in thepast been limited by the appreciable effects of sky-wave interference(night effect) and consequent distortion of the pulse wave formsreceived. Prior pulse timing circuits and devices have been quitevulnerable to this sky-wave distortion, and vulnerable also to theeiects of noise in the output of the Loran radio receiver.

Copending application S. N. 117,917 filed September l5, 1949, by WalterN. Dean on Pulse Synchronizer discloses semi-automatic apparatus forindicating the time delay between a pair of recurrent pulses. Theapparatus disclosed by Dean is a large time-constant servo synchronizersystem which is controlled by the rate of increase in the magnitude ofthe recurrent pulses and hence it is substantially free from the effectsof sky-wave distortion and random noise signals. However, the apparatusdisclosed by Dean must be adjusted by an operator -or the operator mustmake a time measurement on the screen of a cathode-ray tube in order tomeasure the time- -delay between the two recurrent pulses.

Copending application S. N. 131,684 filed by Philip W. Crist on the samedate as the present application discloses and claims the slopecorrection circuit disclosed in the present application.

The present invention is a pulse synchronizer which is an improvementover the synchronizer disclosed by vDean providing completely automaticapparatus for measuring the time-delay between pairs of Loran pulses. ina preferred embodiment, a large time-constant servo system which iscontrolled by the rate of increase in the 'magnitude of the respectiveLoran pulses is employed to ellect synchronization, and precise controlof the syn- -chronization is achieved by employing a variable delaycircuit for introducing a delay in the servo system which variesinversely and substantially exponentially with a control voltage whichhas a magnitude which varies di- 'rectly in accordance with the rate ofincrease in the magarent nitude (i. e., slope) of the respective Loranpulses at the times when the respective Loran pulses attain apredetermined amplitude. Completely automatic operation is attained byemploying in the servo system a servomotor which is caused to rotate anamount proportional to the time-delay between the Loran pulses. Thetime-delay is determined by observing an indicator which shows theamount of rotation of the servomotor.

Accordingly, it is an object of this invention to provide a completelyautomatic Loran receiving system of correct accuracy and reliabilitywhich is not aiected by sky-wave interference or by changes in theamplitude of the pulses received or by random noise signals in theoutput of the Loran receiver.

Another object of the invention is to provide automatic pulsesynchronization apparatus which is responsive to the rate of increase inthe magnitude of each of the control pulses.

Further objects and advantages of the invention will be apparent fromthe following description, the appended claims and the drawings, inwhich:

Fig. 1 is a block diagrammatic disclosure of the completely automaticsynchronizer apparatus showing how it may be employed in a Loranreceiving system;

Fig. 2 shows various curves representing the wave form of and timerelationships between signals which are produced in the various parts ofthe apparatus shown in Fig. l;

Figs. 3 and 4 show two alternative embodiments of the frequencycontrolled oscillator and pulse generator shown in block diagrammaticform in Fig. l;

Fig. 5 shows a schematic diagram of the slope controlled delay circuitshown in block diagrammatic form in Fig. l;

Figs. 6 and 7 are two curves illustrating the operation of the apparatusshown in Fig. 5; and

Figs. 8 and 9 show alternative embodiments of the slope correctioncircuit shown in the block diagram of Fig. 1.

In the discussion of the preferred embodiment of this invention whichfollows, frequent reference will be made to Fig. 2 which shows the waveform of and time relations between the various signals which occur inthe apparatus disclosed in Fig. 1. It is to be observed that the letterwhich identities each curve in Fig. 2 is also employed in Fig. 1 toidentify the circuit which conveys the corresponding signal.

Referring now to Figs. l and 2, the Loran receiver lil serves to receiveand detect the master and slave signals produced by a Loran transmittingsystem (not shown). The detected master pulses are produced at A and thedetected slave pulses are produced at I.

The wave forms shown for the master pulses A and the slave pulses l areillustrative of the wave forms actually received under three normaloperating conditions in which the receiving apparatus is located avariable distance from the master and slave transmitters of the Loransystem. The first master pulse A and the third slave pulse I are notaffected by sky-wave interference. The second and third master pulses Aand the rst and second slave pulses J are aected by sky-waveinterference which distorts the trailing edges of these pulses. Thedierence in the amplitudes of the pulses results from differences in thedistance between the Loran receiver and the transmitters of the Loransystem and to some extent from.

sample the magnitude of the pulses A during each of the gates E whichare applied to' the control circuit thereof, thereby producing outputsignals having magnitudes which are substantially equal to the magnitude:of the pulses A during each of the gates E. The Atilt-er y1 4, whichmay be a conventional type employing lumped cons-tants, has a longtime-constant With respect to the repetition r-ate of the receivedmaster pulses A. The frequency controlled `oscillator and pulsegenerator 16 may be either of the types shown in Figs. 3 and 4. rlhegate generator 18 may be a conventional 'type and it serves to produce1a negative gate B having a duration of the order of the duration ofeach y of Ithe master pulses A.

Fig. 3 shows a frequency controlled 'oscillator and pulse generator 16which is primarily adapted for use in Loran systems in which the pulseshave a single repetition rate. A reactance tube circuit 22, which isresponsive to the output of the lter 14, is employed to vary thefrequency of a crystal controlled oscillator 24 over a small range. Thefrequency of the output of oscillator 24 is reduced to the repetitionrate of the master pulses A by a divider 26 which may be a conventionaltype, and the output of the divider 26 is applied to a pulse generator28 which serves to produce a pulse of short duration in response to eachcycle of the output of the divider 26.

The alternative frequency controlled oscillator and pulse generator 16shown in Fig. 4 is pramarily adapted for use in Lor-an systems in Whichmore than one transmitter system is employed and some of the transmittersystems produce pulses having a repetition rate which diiers from therepetition rate of the pulses produced by the other ltransmittersystems. It will be understood that the generator shown in Fig. `El isinterchangeable with that shown in Fig. 4 for Loran system-s in whichthe repetition rates Vdiffer by moderate amounts. Referring now to Fig.4, .a potentiometer 30 which is connected across a battery 32 serves t-ocomplete the circuit between the iilter M- and a servo amplifier 34, andthe voltage introduced into the servo loop by the potentiometer 30 is ofopposite polarity to the voltage produced by the ilter 14. The output ofthe servo amplifier 34 is applied to a servomotor 36 which serves tocontrol the frequency of an adjustable oscillator 38 over the requiredfrequency range. The frequency `of the output of the adjustableoscillator 33 is reduced to the repetition 4rate of the master pulses Aby a divider 26' which may be the same type as the divider 26, land theyoutput of the diyider 26', is yapplied to a pulse generator 28' whichmay be the same type as the pulse generator 28 and serves to produce apulse of short duration in response to each cycle .of the output of thedivider 26.

An import-ant feature of the slope correction circuit 20 is a slopecontrolled delay circuit 40 which will be described in detailhereinafter. Gates B Iand signa-l G (which has a magnitude which variesin laccordance with the rate lof increase in the magnitude of therespective master pulses A) are applied to 'the slope controlled delaycircuit 4@ `and signals C are produced thereby which are delayed a timeafter the leading edges of the respective gates B. The time-delay variessubstantially inversely and exponentially with the magnitude of thesignal G.

The signals C are amplified by amp1ier42, the amplii'ied signals aredifferentiated by a conventional diierentiator 44, the differentiatedsignals are clipped by a conventional clipper 46, and the output D ofthe clipper 46 is employed to actuate a conventional ,gate generator 48lto produce gates E which are of very short duration with respect to theduration of each of the master pulses A.

The gates E are applied to pulse wave coincidence circuit 12 and serveto actuate this circuit ,during each gate E so that the magnitude of thesignal A is sampled during cach of the gates E. The circuit constants ofthe lfirst servo loop are proportioned so .that the gates E are causedto occur when the respective master pulses attain a predeterminedmagnitude'such as two volts lfor example. If the frequency controlledoscillator and pulse generator 16 shown in Fig. 3 is employed, this maybe accomplished by adjusting the fixed bias voltage on the grid of thereactance tube. If the frequency cont-rolled oscillator and pulsegenerator 16 shown `in Fig. 4 is employed, this may be accomplished byadjusting the potentiometer 30.

The gates E are also applied to the pulse wave coincidence circuit 50which may be the same type =as ,crcuit l2. Also, a dilerentiated versionF of the master pulses A is applied to the pulse wave coincidencecircuit 50 by means of a diierentiator 52 and 'an Iamplifier 54, both ofwhich may be conventional types.

The gates E serve to actuate the pulse wave coincidence circuit Sil sothat the magnitude of the signal F is sampled during each of the gate-sE. The `output of the pulse Wave coincidence circuit 50 is applied @to afilter 56 which preferably hlas a long time-constant with respect to thetime-constant :of the iilter 14 and produces a signal G which has amagnitude which varies directly in accordance with the rate of increasein `the magnitude of the respective master pulses A during each gate E.Signal G is `applied to the control circuit of delay circuit 40 @andserves to control the delay introduced by the slope controlled delaycircuit 40.

Since the signals C are caused to occur when the re spective masterpulses A attain `a predetermined amplitude such as two volts, it followsthat the gates B are caused to occur a time before the respective masterpulses attain the amplitude of two volts. It has been found that if Ithe,time-delay 6 is caused to vary exponentially and inversely with therate of increase in the magnitude of the respective master pulses A iatthe times when the master pulses A attain a certain magnitude such astwo volts, the true starting point of the master pulses A may bedetermined by subtracting the time from the time at which the masterpulses A attain Aa magnitude of two volts. Since 4the leading edges ofgates B are caused to occur a time before signals C, it follows that theleading edges of gates B are precisely coincident with the leading edgesof the master pulses A.

The slope controlled delay circuit 40 shown in Fig. 5 serves to producethe signals C which serve to introduce the time-delay required.

Referring now to Fig. 5, triode tubes 58 and 60 are employed Ato producevoltage drops across their respective cathode resistors 62 and 64 whichare compared in diode 66 to produce signals C when the voltage betweenground and the tap 6 7 on resistor 64 exceeds the voltage acrossresistor 62.

Tube 58 is normally supplied with positive grid voltage by means of abattery 68 and a resistor 70. Thus, the tube 5S is normally in aconducting condition and a voltage drop is produced across the cathoderesistor 62. `When each negative gate B occurs, the tube 5S is biased tocut-off for the duration of the gate and the voltage across thecondenser 7 2 decays exponentially due to the discharge of the condenser72 through resistor 62. The control signal G is applied across a gridresistor 74 and serves to apply a positive voltage to the grid of tube60 so that tube 6o is in a conducting condition and a current owsthrough cathode resistor 64. A portion of the voltage drop acrossresistor y6 4 is lapplied to the plate ot' tube 66 through resistor 76and the lap 6 7 on resistor dit. Resistor 62'and the tape 6 7 onresistor 64 are adjustable so that the time-,delay introduced by circuit40 may be adjusted.

Thus the signals produced across the resistors 62 and 64 are compared bythe diode 6 6, when the voltage from the plate of diode 66 to groundexceeds the voltage from the cathode of diode 66 to ground, currentflows from resistor 64 through resistor 7 6, diode 66 and sig nais C(Fig. `7) are produced which, by exponential decays of an otherwiseconstant voltage each time that the Triodes 58 and 60 Type 12AT7. Diode66 Type 6AL5. R62 15,000 ohms. R64 5,000 ohms. R70 24,000 ohms. R74 10megohms. R76 51,000 ohms. C72 .0036 mf.

C78 100 mmf. Battery 68 25 volts.

The gates B areV applied to a conventional diierentiator 79 whichproduces a signal H which is applied to conventional-clipper andinverter 80 which in turn produces a series of pulses I, each of whichoccurs in xed time relation with respect to the instant when therespective master pulses A are initiated.

The slave pulses I are applied to a second servo loop which comprises apulse wave coincidence circuit 12', a iilter 1d', a potentiometer 81 anda battery 82 serially connected therewith, a servo amplifier 84, aservomotor 86, a potentiometer 88 and a battery 90 serially connectedtherewith, a Variable delay circuit 92 and a portion of the slopecorrection circuit The pulse wavev coincidence circuit 12', the filter14 and the slope correction circuit Ztl may be the same as the circuitelements 12, 1d, and 20 described above. The servo amplifier 84 and themotor 86 may be conventional types. The variable delay circuit 92 may bea variable-delay one-shot multivibrator such as the type disclosed onpage 591 of the book Electronic Instruments, by Greenwood, Holdarn andMacRae, published by the McGraw-Hill Book Company in 1948. The circuit92 serves to produce a positive gate K which has a duration which variesdirectly with the magnitude of the control voltage produced by thepotentiometer 88.

Paises i are applied to the input of the variable delay circuit 92 andserve to initiate the respective gate pulses K produced thereby. Thegates K are applied to the slope controlled delay circuit 40 whichproduces a signal L which decays exponentially at times after thetrailing edges of gates K. It will be observed that the slope controlleddelay circuit 40 is actuated by the trailing'edges of the positive gatesK, Whereas circuit 40 is actuated by the leading edges of the negativegates B: This is due to the fact that circuit 40' must be actuated by anegative gate and the portions of signal K between each positive gateare employed as negative gates for circuit dii'. Signals L areamplified, diierentiated 'and clipped to produce pulses M which'areapplied to gate generator d8 which in turn produces gates N which are'ot very short duration with respect to the duration of slave pulses J.The gates N serve to actuate pulse wave coincidence circuit 12 so thatcircuit l2 produces signals which are proportional to the magnitude ofthe slave pulsesl I during each gate N. This signal is applied to. thev[ilter i4 and the output of the filter 14 is applied to the servoamplier d4 through the Search bias poteri-v tiometer 8l. The voltageintroduced into the servo loop by the potentiometer 31 is of oppositepolarity to the voltage produced by the iilter 14. The output of theservo amplifier 8d is employed lto actuate the motor 86 which in turncontrols the position of the potentiometer 88. The output of thepotentiometer 88 is applied to the control circuit of the variabledelay; circuit 92 and serves to control the duration of each gate pulselK.

- The second servo loop is adjusted by means'of the potentiometer S1 sothat each gate N is caused to occur when the respective slave pulses Jattain a predetermined magnitude such as two volts.

The slave pulses J are diierentiated by diierentiator 52' and amplifiedby amplifier 54 to produce signals l which are sampled by pulse wavecoincidence circuit 50 to produce a variable voltage P at the output ofiilter 56 in the same manner as signal G is produced by slope correctioncircuit 20. As before, signal P has a magnitude which varies inaccordance with the rate of increase in the magnitude of the respectiveslave pulses J at the time that the respective slave pulses J attain anamplitude of two volts. Signal P is employed to control the delayintroduced` by slope controlled delay circuit d0 as discussed above withreference to circuit 40.

The circuit constants of the slope controlled delay circuit 40' areproportioned so that the delay introduced thereby is precisely equal tothe difference in time between the time when the respective slave pulsesJ are initiated and the time when the slave pulses I attain a magnitudeof two volts, as discussed above with reference to slope controlleddelay circuit 40. In this manner the trailing edges of gates K arecaused to occur precisely at the instant when the respective slavepulses I are initiated. Thus, the duration of lgates K is a measure ofthe time-delay t between the respective master and slave pulses. Sincethe duration of gates K is determined by the voltage produced bypotentiometer 88 which in turn is determined by the position of therotor of motor 86, it follows that the time-delay l between the masterpulses A and the slave pulses I is determined automatically by anindicator 94 which shows the position of the rotor of motor 86.

Figs. 8 and 9 show two alternative slope correction circuits 96 and 97which may be employed in the place of the slope correction circuit 20 or20 shown in Fig. l.

The slope correction circuit 96 diiers from the circuit 20 in that noamplifier is employed between the differentiator 52 and the pulse wavecoincidence circuit 50 and in that the gates produced by means of delaycircuit 98 and gate generator 99 which serve to actuate the pulse Wavecoincidence circuit 50 are delayed a xed time after the respectivepulses M. In this manner, the differentiated pulses are sampled by thepulse wave coincidence circuit 50 a fixed time after the respectiveslave pulses I attain la predetermined amplitude such as two volts,thereby providing a measure ofthe rate of increase in the magnitude ofthe pulses.

The slope correction circuit 97 shown in Fig. 9 differs from the circuit96 shown in Fig. 8 in that the slave pulses I are introduced directly tothe pulse wave coincidence circuit 50. As before the pulse wavecoincidence circuit 50 is actuated a fixed time after the pulses M asdetermined by the delay introduced by delay circuit 98. Thus, sincepulse Wave coincidence circuit 50 is actuated a lixed time after therespective slave pulses J attain a predetermined amplitude, it followsthat the signal produced by circuit 50' varies in accordance with therate of increase in the magnitude of the respective slave pulses Iduring the delay time introduced by circuit 98.

Itwill be observed that in the apparatus shown in Fig. l, the master andslave pulses are sampled during the leading edges of the respectivepulses so that the sampling process is not aiected by sky-waveinterferencewhich, in conventional Loran systems, usually occurs. about50 microseconds after the direct wave signals are; received.

Furthermore, it will be observed that the various servo;

it will be apparent that various modications may be:

made in the Aapparatus disclosed herein. For example,

various types' of well-known circuits may be vemployed instead of thefour diode type pulse wave coincidence cire cuits 172, l2', 50 and 50'Aor instead of the variable-delay oneshot multivibrator type variabledelay circuit 92 d escribed herein. i

Since many changes could be made in the above construction and manyapparently widely diierent embodiments of this invention could be madeWithout departing from the scope thereof, it is intended that all mattercon tained in theabove description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

l. in combination, a sampling circuit having an input circuit adapted tobe connectedy to a` first source of recurrent pulses, a iirst variabledelay means responsive to the output of said sampling circuit and havingan input circuit adapted to be connected Ato a second source ofrecurrent pulses for producing signals delayed with respect to thepulses produced by said'secondsource a time which varies in accordancewith the magnitude of the signal produced by said sampling circuit, asecond variable delay circuit responsive to the output of said iirstdelay circuit and having a control' circuit connected to said inputcircuit for producing signals delayed with respect to the signalsproduced by said first delay circuit a time which varies inversely inaccordance with the rate of increase in the magnitude of the pulses ofsaid first series, and means for actu-ating said sampling means inresponse to the signals produced byl said second delay circuit.

2. The apparatus of claim l further including means for indicating thetime-delay introduced by said first variable delay circuit.

3. in combination, a sampling circuit having an input circuit adapted tobe connected to a source of recurrent control pulses, a servomotorresponsive to the output of said sampling circuit, means for producing asignal which varies in accordance with the position of the rotor ofysaid motor, a first variable delay means responsive to said signal andhaving an input circuit adapted to be connected to a second source ofrecurrent pulses for producing signals delayed with respect to thepulses produced by said second source a time which varies in accordancewith the magnitude of the signal produced by said sampling circuit, asecond variable delay circuit responsive to the output of said firstdelay circuit and having a control circuit connected to said inputcircuit for producing signals delayed with respect to the signalsproduced by said rst delay circuit `a time which varies inversely inaccordance with the rate of increase in the magnitude of the pulses ofsaid first series, and means for actuating said sampling means inYresponse to the signals produced by said second delay circuit.

4. The apparatus of claim 3 further including means for indicatingtheposition ofi-the rotor of said motor and thereby indicating thetime-delay between said control pulses andsaid series of signals.

5. A servo system comprising a pulse Wave coincidence circuit havinginput, output and control circuits, a servomotor responsive to theoutput of said coincidence circuit, means for producing a control signalwhich variesin accordance with the position of the rotor of said motor,a first variablel delay circuit having input, output and controlcircuits, the control circuit of said delay circuit being connected tosaid control signal producing means and serving to cause said firstydelay circuit to produce output signals delayed with respect to signalsapplied to saidY inputA circuit a time which varies in accordance withthe magnitude of said control signal, and a second variable delaycircuit connected between the output of said firstv delay circuit andthe control circuit of said pulse vvave coincidence circuit, saidvsecond variable delay circuit having a control circuit connected to theinput of said pulsey waveA coincidence circuit and serving to` introducea time-delay between its inputand output circuits-` which is inverselyalle substantie@ @essentially rrreuigal to.

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the rate of increase of the signals applied to the input circuitv ofsaid'pulse Wavev coincidence circuit.

6.` In combination, an input circuitV adapted to be con nectevd to asource of recurrent control pulses, means connected to said inputcircuit for instantaneously sampling the magnitude of said controlpulses, a filter connected to the output of said sampling means, aservomotor responsive to the, output of said filter, means for producinga control signal which varies in accordance with the position of therotor of said motor, means for producing a series' of vpulses having arepetition rate harmonically related to the repetition rate of saidcontrol pulses, av rst variable delay circuit responsive to said controlsignal and to said series of pulses for producing signals delayed aftereach pulse or" said series a time which varies in accordance with` themagnitude of said control signal, means connected vto Ysaid inputcircuit for pro` ducing a control voltage having a magnitude whichvaries in accordance with theA rate of increase in the magnitude of thecontrol pulses, a second variable delay circuit responsive to the outputof said first delay circuit and to said control voltage for delaying thesignals produced by said first delay circuit a time which varies inaccordance with the magnitude of said control voltage, and meansinterconnecting the output of said second delay circuit and the controlcircuit of said sampling means for actuating said sampling means inresponse to the delayed pulses.

7. ln a radio system employing a pair of transmitters to. produce afirst and second series of pulse-modulated electromagnetic waves havingpredetermined time relationships, a receiver. adapted to= detect saidWaves and produce signals corresponding to said first and second seriesof pulses, means responsive to the output of said receiver` forproducing a series of pulses synchronized with. saidl first series ofpulses, means responsive to the output of said receiver forinstantaneously sampling the magnitude` of the pulses of said secondseries of pulses, a rst variable delay circuit responsive to the outputof said sampling means and to said series of synchronized. pulses, asecond variableY delay circuit responsive to the output of said rstdelay` circuit and having a control circuit responsive to. said secondseries of pulses for producing signals delayed with respect to thesignals produced by said tirst delay circuit a time which variesinversely in accordancewith the rate of increase in the magnitude of thepulses of said second series, and means for actuating said samplingmeans in response to the signals produced by said second delay circuit.

S. The apparatus of claim 7 further including means for. indicating thetime-delay introduced by said first variable` delay circuit.

9. In a radio system employing a pair of transmitters to produce a firstand a second series of pulse-modulated electromagnetic waves havingpredetermined time relationships, a receiver adapted to detect saidwaves and produce signals corresponding to said first and second seriesof pulses, means responsive to the output of said receiver' forproducinga seriesv of pulses synchronized with said rst series of pulses, meansresponsive to the outputy of said receiver for instantaneously samplingthe magnitude of the pulses of said second series of pulses, aservomotor responsive to the output of said sampling means, means forproducing a control signal having a magnitude which varies in accordancewith the position of the rotorof said motor, a first variable delaycircuit responsive to said signal and to said series of synchronizedpulses for producing signals delayed after said synchronized pulses atime which varies in accordance with the magnitude of saidv controlsignal, a second variable delay circuit responsive to the output of saidfirst delay circuit and having a control circuit connected to the outputof said receiver and responsive to said second series of pulses for,producing signals delayed with respect to the signals produced by saidtirsft delay circuit a 2,778,01 1 9 10 time which varies inversely inaccordance with the rate dicator responsive to the position of the rotorof said of increase in the magnitude of the pulses of said second motorfor showing the time-delay between said rst and series, andinterconnecting means between the output of said second series ofpulses. said second delay circuit and the control circuit of saidsampling means for actuating said sampling means in 5 References Citedinthe me of this Patemt response to the signals produced by said seconddelay UNITED STATES PATENTS Circuit- 2,497,513 Pain et a1. F b, 14 195o10. The apparatus of claim 9 further including an m- 2,523,244 WooiwardSeit 19 1950

